1. Field of the Invention
The present invention relates generally to a method and system for automatically attaching a sub-assembly to a main assembly using two industrial robots and, particularly, relates to a method and system for automatically attaching a sub-assembly such as a strut assembly of a strut-type front suspension to a predetermined position of a main assembly such as a strut tower of a vehicle body by means of fastening means such as a nut and repeating such an operation whenever the main assembly is transported on a conveyor belt.
2. Description of the Prior Art
Conventionally, a plurality of operators carry out manually such an assembly operation in an assembly line of, e.g., automotive vehicles as attaching a sub-assembly, e.g., a strut assembly of a strut-type front suspension (hereinafter, simply referred to as a strut) to a main assembly, e.g., a vehicle body whenever the main assembly, i.e., vehicle body is transported on a conveyor belt.
In other words, one of the plurality of operators carries one of the struts stored in a bin to the vehicle body to fit an upper extended portion of the strut into a positioning hole located at a predetermined position of the vehicle body so as to position the strut and holds the strut at the predetermined position, maintaining the strut fitted into the positioning hole. Subsequently, another operator carries a nut fastening tool to the predetermined position and manipulates the nut fastening tool to fasten at least two nuts into threaded portions of corresponding bolts projected from the strut which have previously been inserted through corresponding bolt inserting holes provided at predetermined positions of the vehicle body. In this way, the attaching of a single strut to the predetermined position of the vehicle body is completed.
However, it is troublesome for the operators to carry out the above-described assembly operation since a considerably long working time is required and, in addition, the strut is relatively heavy for carrying it to the vehicle body to attach to the vehicle body.
Therefore, there arises an industrial demand for automating such assembly operations without intervention of these operations.
A method for automatically attaching the strut to the vehicle body includes the utilization of industrial robots. Such a method, for example, comprises the following steps of: (a) positioning the vehicle body on the conveyor belt; (b) grasping and carrying the strut using one of two robots to place the upper engagement portion of the strut at the predetermined position of the vehicle body while carrying a nut fastening tool holding fastening means using the other robot to the predetermined position of the vehicle body at which the strut is fixedly attached to the vehicle body with the nuts. In the step (b), each of the robots carries the strut and fastening tool at the predetermined position on a predetermined trajectory determined by pre-programmed position teachings of a robot control unit, such as a microcomputer.
However, there is a problem in the above-described method that the strut and fastening tool are placed at a position deviated from the predetermined position at which the strut carried by the one robot is to be attached to the vehicle body by means of nuts held by the fastening tool attached on the other robot since the accuracy of positioning the vehicle body at a predetermined stop position is relatively low from the standpoint of the present control technology. Therefore, although it is necessary to carry the strut at a position required for attaching the strut to the vehicle body and also to position the fastening tool at the position required for attaching the strut to the vehicle body, it requires more time to have the strut and fastening tool positioned with respect to the vehicle body separately. Consequently, the total working time for securedly attaching the strut to the vehicle body cannot be reduced. In addition, if the robots are adapted to play the roles of positioning strut and fastening tool, a control unit storing pre-programmed instructions on which the robots move the strut and fastening tool to locate at their respectively required positions with respect to the vehicle body is additionally required so that the whole system for automatically attaching the strut to the vehicle body becomes complicated.
It should be noted that such problems arise not only in the case where the strut is attached to the vehicle body but also arises in such cases where other workpieces are attached to a main workpiece which is transported on the conveyor belt.